You can put this solution on YOUR website! Find the center and radius of each circle:
x2 + y2 = 36
The standard form for a circle with center at (h,k) and radius r is:
(x-h)^2 + (y-k)^2 = r^2
Since x^2 + y^2 = 36 is the same as:
(x-0)^2 + (y-0)^2 = 6^2
this cicle has center (0,0) and radius 6.